The present invention relates to a manufacturing method of an indirectly heated cathode to be used as an electron tube such as a cathode ray tube. When using an indirectly heated cathode as cathode ray tubes, such as a picture tube of a television or a display tube of an information processing apparatus, it is desired that the time required for the appearance of the picture on the display screen from the time of switching on, due to the thermoelectronic emission caused by the rise of temperature of the cathode, be reduced as far as possible.
An indirectly heated cathode is described in Japanese Patent Laid Open No. 51-50564, wherein the cathode has a structure which includes a cap having thermoelectronic emission material adhered thereto, covering the top of the cathode sleeve and a heater inserted into the inside of the cathode sleeve used to heat the thermoelectronic emission material. In this indirectly heated cathode the emission warm up time of the thermoelectronic emission can be reduced by providing the black coating on both the internal and external surfaces of the sleeve.
When the black coatings are provided on both the internal and the external surfaces of the cathode sleeve, however, the radiation of the heat from the external surface of the cathode sleeve increases thereby causing an increase in the power consumption of the cathode tube. The increase in the power consumption by the cathode system causes an increase in temperature in the electron tube, which results in a thermal transformation of the electrodes, the occurrence of stray emission due to the rise of temperatures of the parts of the electrodes and deterioration of the electron tube as the whole.
When no black coatings are provided to either the internal or the external surfaces of the cathode sleeve, therefore eliminating the aforementioned adverse effects, a decrease in the efficiency, at which the internal surface of the cathode sleeve absorbs the heat radiation from the heater, occurs, resulting in an increase in the emission warm up time of the thermoelectronic emission.
Thus, in order to obtain an indirectly heated cathode with small power consumption and short emission warm up time of the thermoelectronic emission, the black coating should be provided only on the internal surface (on the side of the heater) of the cathode sleeve.
Examples of electron tubes to which black coating is applied only to the internal surface of the cathode sleeve, are described below. To enable the realization of a low-power-consumption indirectly heated cathode a dual cathode sleeve is described in Japanese Patent Laid Open No. 53-145464. However this technique is disadvantageous due to an increase in the number of parts and assembly steps needed. This causes not only an increase in the thermal capacity of the cathode sleeve itself, and a resultant increase in the emission warm up time of the thermoelectronic emission, but also an increase in the production cost. One method for providing the black coating only to the internal surface of the cathode sleeve without using a dual construction for the cathode sleeve, is to provide black coatings to both the internal and the external surfaces of the cathode sleeve by an ordinary process (For example, heat treatment in wet hydrogen), then to remove the black coating on the external surface by barrel finishing. However, this method has a disadvantage in the possibility of having the cathode sleeve deformed during the barrel finishing, thus adversely affecting the quality control of the manufacturing process. As proposed in Japanese Patent Laid Open No. 48-66968, there is a method featuring the deposit of tungsten powder on both the internal and the external surfaces of the cathode sleeve for facilitating the absorption of heat. However this method has the disadvantage of requiring firing of the dried coating of the tungsten suspension in a reducing atmospher, which is a drawback to mass production efficiency. Further as stated previously, having the black coating on both the internal and the external surfaces of the cathode sleeve causes not only an increase in power consumption but also the occurrence of stray emission which results in deterioration of the characteristics of the electron tube. Moreover, the method proposed in said Japanese Patent Laid Open No. 48-66968 includes spraying the mixture of tungsten and aluminium oxide, and firing the coating in a reducing atmosphere to form the black coating. Therefore there is a possibility that the black coating will be exfoliated due to contact of the black coating with the heater inserted in the cathode sleeve and the thermal stress caused by the repetition of the on-off action. Also, the electron tube manufactured by this method has the disadvantages that the electron tube will have a large thermal capacity due to the black coating having a thickness of more than several micrometers, with a resultant increase in the emission warm up time, and a reduction in design allowance, since a reduction in the inside diameter of the cathode sleeve will require a reduction in the size of the heater to be inserted into the cathode sleeve.